1. Field of the Invention
The present invention generally relates to surface acoustic wave filters, and, more particularly, to a surface acoustic wave filter used in a high-frequency circuit of a wireless device such as a portable telephone.
2. Description of the Related Art
In general, surface acoustic wave devices are widely used as filters in high-frequency circuits of wireless communication devices such as portable telephones. FIG. 1 is a block diagram of a high-frequency terminal of a portable telephone that comprises surface acoustic wave devices as a reception filter 4 and a transmission filter 8.
On the reception side, a signal inputted through an antenna 1 is filtered by a branching filter 2 so as to obtain a signal having a certain frequency. The filtered signal is then subjected to amplification in a low noise amplifier 3, and supplied to the reception filter 4 constituted by a surface acoustic wave device. The signal is subjected to passband restriction in the reception filter 4, and then superimposed on a carrier generated by a local oscillator 6 in a mixer IC 5A. The superimposed signal is transmitted to an intermediate frequency unit. On the other hand, a transmission signal from a modulator is superimposed on a carrier generated by the local oscillator 6 in a mixer IC 7. The superimposed signal is then subjected to passband restriction in the transmission filter 8, and then amplified by a power amplifier 9. The amplified signal is then filtered by the branching filter 2, and transmitted through the antenna 1.
In recent years, in such a high-frequency circuit of a wireless communication device, a mixer IC having balanced input and output, or differential input and output has been used. FIG. 2 is a circuit block diagram of the high-frequency unit of a portable telephone having a balanced mixer IC 5B. As shown in FIG. 2, the balanced mixer IC 5B comprises a pair of input terminals 11A and 11B. Using the balanced mixer IC 5B, an adverse influence from noise can be reduced, and the output can be stabilized. Thus, the characteristics of the portable telephone can be improved.
However, the conventional surface acoustic wave filter, which constitutes the reception filter 4, requires a balance-unbalance conversion transformer 10 or an independent conversion circuit that performs balance-unbalance conversion between the reception filter 4 and the balanced mixer IC 5B, because the input and output terminals of the reception filter 4 are unbalanced.
Furthermore, the surface acoustic wave filter, which constitutes the reception filter 4, has a normal impedance of 50xcexa9, while the balanced mixer IC 5B, which comprises the balanced input terminals 11A and 11B, has a higher impedance of 100 to 200xcexa9. Therefore, an impedance conversion circuit is also required to connect the reception filter 4 and the balanced mixer IC 5B.
For the above reasons, with the balanced mixer IC 5B, the characteristics of the portable telephone can be improved, but the number of components is increased. As a result, the demands for smaller, lighter, and less expensive portable telephones cannot be satisfied.
A general object of the present invention is to provide surface acoustic wave devices in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a surface acoustic wave device that has an unbalance-balance conversion function and an impedance conversion function.
The above objects of the present invention are achieved by a surface acoustic wave device that includes:
a piezoelectric substrate;
a first surface acoustic wave filter that is formed on the piezoelectric substrate, and has at least one input interdigital transducer and at least one output interdigital transducer arranged alternately on a surface acoustic wave path on the piezoelectric substrate; and
a second surface acoustic wave filter that is formed on the piezoelectric substrate, and has at least one input interdigital transducer and at least one output interdigital transducer arranged alternately on the surface acoustic wave path on the piezoelectric substrate, the phase difference between the first surface acoustic wave filter and the second surface acoustic wave filter being approximately 180xc2x0.
In this surface acoustic wave device, the input interdigital transducers of the first surface acoustic wave filter and the second surface acoustic wave filter are electrically connected, or the output interdigital transducers of the first surface acoustic wave filter and the second surface acoustic wave filter are electrically connected. A terminal extending from the connecting point between the first and second surface acoustic wave filters serves as an unbalanced terminal. Among the interdigital transducers disposed to the first and second surface acoustic wave filters, those unconnected between the first and second surface acoustic wave filters have terminals extending therefrom, and the terminals serve as a balanced terminal.
According to the present invention described above, the surface acoustic wave device has the unbalanced terminal on the input side and the balanced terminal as the output side. With such a structure, a circuit or component that performs balance-unbalance conversion is no longer required. Accordingly, the number of components, the size, the weight, and the cost of an electronic apparatus (such as a portable telephone having a balanced mixer IC) can be reduced.
The above objects of the present invention are also achieved by a surface acoustic wave device that includes:
a piezoelectric substrate; and
five interdigital transducers that are disposed on a surface acoustic wave path on the piezoelectric substrate.
Among the five interdigital transducers, the first, third, and fifth interdigital transducers counted from one end of the device are input interdigital transducers and electrically connected at electrodes on one side, and a terminal extending from the connecting point between the first, third, and fifth interdigital transducers serves as an unbalanced terminal. On the other hand, the second and fourth interdigital transducers counted from the one end of the device serve as output interdigital transducers and are electrically connected at electrodes on the other side. The phase difference between the electrodes on the one side and the electrodes on the other side of the second and fourth output interdigital transducers is 180xc2x0. A first terminal extends from the connecting point between the electrically connected electrodes on the one side of the second and fourth interdigital transducers, while a second terminal extends from the connecting point between the electrically connected electrodes on the other side of the second and fourth interdigital transducers. The first terminal and second terminal constitute a balanced terminal.
The surface acoustic wave device of the present invention has an unbalanced terminal on the input side and a balanced terminal on the output side. Accordingly, a circuit or component that performs balance-unbalance conversion is no longer required. Thus, the number of components, the size, the weight, and the cost of an electronic apparatus (such as a portable telephone comprising a balanced mixer IC) having the surface acoustic wave device mounted thereon can be effectively reduced.
Furthermore, with the surface acoustic wave device including the five interdigital transducers (5-IDT filter), a stable characteristics can be obtained in a wider passband.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.